1. Field of the Invention
The present invention relates to an optical scanner used for an image recording device such as a laser printer and a digital copying machine for recording an image by exposing and scanning a scanned medium according to image information using a laser beam.
2. Description of the Related Art
Generally, an optical scanner is composed of a semiconductor laser beam source element that generates a light beam, a deflecting system, an optical component such as an imaging and scanning lens and a reflector and an optical box for housing them and is fixed to an image formation device by screws.
Generally, an optical box is fixed to an image formation device at three or four points as shown in FIGS. 22 and 23.
In FIGS. 22 and 23, a reference number 102 denotes an optical box and 104 denotes a fixed position.
If an optical box is fixed at four points as shown in FIG. 23, it is fixed in the vicinity of a vertex of its outside shape and if an optical box is fixed at three points as shown in FIG. 22, at least two points of them are also provided in the vicinity of a vertex of its outside shape.
For an optical scanner for an image formation device corresponding to a recent tendency to color, the one having plural light sources and plural scanning optical systems and housed in one optical box is disclosed in Japanese Published Unexamined Patent Application No. Hei 4-127116.
An optical box 106 of an optical scanner 105 shown in FIG. 24 is also fixed to a frame 108 of the body of a color image formation device at three points (a reference number 104 denotes a fixed position) and for the fixed position 104, two of the three points are provided in the vicinity of a vertex of the outside shape of the optical box as the above example of a conventional type.
In the optical scanner 105, to meet characteristic values such as a spot diameter on a photoconductor, the quantity of light and the position of a scanning line, the above optical component composing the optical scanner 105 is precisely mounted in the optical box 106, the attitude of a light source and an optical component is regulated and a laser beam optical path is composed and regulated so that the above optical path is ideal.
However, when the optical scanner 105 is mounted on the frame 108 of the color image formation device and an image formation process is operated, temperature around the optical scanner 105 differs from time when the optical scanner is assembled and regulated because of heat generated from a fixing device in the color image formation device and environment in which the color image formation device is installed.
At this time, when there is difference between the coefficient of linear expansion of the material of the frame 108 of the color image formation device and that of the material of the optical box 106, difference occurs in the quantity of expansion between the frame 108 and the optical box 106.
Particularly, the optical box of a recent optical scanner is generally made of resin to reduce the cost.
Therefore, as the optical box 106 made of resin is sufficiently larger than the frame 108 made of sheet metal in the coefficient of linear expansion in case the optical box 106 is made of resin and is fixed to the frame 108 made of sheet metal, large flexure is caused in the optical box 106 made of resin.
FIGS. 25A and 25B schematically show states before and after an optical box 112 made of resin is transformed when temperature in the following image formation device rises in case the optical box 112 made of resin is fixed to the body frame 114 of the image formation device made of metal by screws 116.
When temperature in the image formation device varies, the bottom of the optical box 112 made of resin is warped like a drum as shown in FIG. 25B, as a result, a part in which a rotary polygon mirror 118, an fO lens group 120 and a plane mirror 122 are mounted is displaced, an ideal optical path L is deflected to be an optical path L', characteristic values on a photoconductor produced by an optical scanner cannot be met, in addition, a light beam does not reach the photoconductor and a terrible failure of the quality of an image such as a void may be caused.
Particularly, when the part in which a reflector such as the rotary polygon mirror 118 and the plane mirror 122 is mounted is tilted because of deformation off an ideal state, the optical path is largely influenced. As described above, such a warp in a direction shown by an arrow Z of the bottom of the optical box 112 as the part in which an optical component is mounted is tilted has a large effect upon the variation of the optical path.
As the deformation by a change of temperature of the optical box is caused because of difference in the coefficient of linear expansion, the material of the optical box and the image formation device frame fixing the optical box, the number of points at which the optical box is fixed to the image formation device frame and a fixed position have an effect.
FIGS. 26 to 28 show results that the shape of an optical box in a conventional type of a general optical scanner is modeled tobe a simplemodel and deformation by heat is simulated using a finite element method.
FIG. 26 shows the shapeof asimplemodel. It is premised that an optical box 102 is made of polycarbonate resin and the optical box 102 is fixed to the body frame made of iron of the image formation device not shown by screws, and is performed simulation when temperature in the image formation device rises up to 35.degree. C. in a state in which fixed positions 104 (shown in black circles) are provided in the vicinity of a vertex of the outside shape of the optical box in case the optical box is fixed at four points as in the conventional type and in a state in which two fixed positions are provided in the vicinity of a vertex of the outside shape of the optical box in case the optical box is fixed at three points.
FIGS. 27 and 28 shows the result of simulation respectively showing the deformed states of the optical box in case it is respectively fixed at three points and at four points. Each drawing shows deformation in a direction of the y-axis (shown by an arrow Y) which has a large effect particularly upon the variation of an optical path as described above.
As clear from FIGS. 27 and 28, the bottom of the optical box is warped like a drum between points fixed by a screw (the fixed positions 104).
Next, the shape of an optical box for an optical scanner used for a color image formation device is modeled to be a simple model and the simulation of deformation due to heat is performed.
The optical box for the optical scanner for the color image formation device is generally made of metal in view of size, precision and strength.
Therefore, simulation when temperature in the image formation device rises up to 35.degree. C. in case the optical box is made of aluminum, the body frame of the image formation device is made of iron and the optical box is fixed at three points as in the conventional type is performed.
FIG. 29 shows the shape of a simple model of an optical box 106, and FIGS. 30 and 31 show the result of simulation respectively showing a deformed state.
As shown in FIG. 31, the bottom of the optical box 106 is warped like a drum between points fixed by a screw (fixed positions 104) as the result of a general optical scanner.
A mode of deformation differs between right and left parts in case the above bottom is divided based upon a central line C--C of the optical box 106.
In the optical scanner of the color image formation device, a positional characteristic of each light beam is very important in addition to characteristic values when a general single light beam scans a photoconductor.
In the color image formation device, a light beam corresponding to each color from the optical scanner is irradiated and scans on each photoconductor 130K, 130Y, 130M, 130C corresponding to each color of yellow (Y), magenta (M), cyan (C) and black (BK) as shown in FIG. 32, an electrostatic latent image is formed and is developed by each developing device 132K, 132Y, 132M, 132C.
Afterward, transfer paper not shown on a transfer belt 134 is overlapped with the image, the image is transferred on the paper via an intermediate transfer roller and is fixed by a fixing device 138.
Therefore, if the deformation due to the change of temperature of an optical box 106 differs in the right and left parts in an optical scanner provided with an optical system corresponding to right and left each color with a rotary polygon mirror 139 in the center in one optical box 106 such as a conventional type optical scanner, the quantity of the variation of the optical path of each scanning line differs among each optical path, a scanning position (a relative position) of each light beam is displaced, as a result, color misregistration is caused and the quality of a color image is greatly influenced.